The polyphenylene oxides and processes for their preparation are known in the art and are described in numerous publications, including Hay, U.S. Pat. No. 3,306,874 and U.S. Pat. No. 3,306,875. Other procedures are described in the Bennett and Cooper patents U.S. Pat. Nos. 3,639,656; 3,642,699; 3,733,299; 3,900,445 and 3,661,848. All of these patents are hereby incorporated by reference.
The processes most generally used to prepare polyphenylene ethers are based on the use of a copper-amine complex catalyst and oxygen in the self-condensation of a monovalent phenol. Various techniques have been employed in terminating this polymerization reaction, including the use of aqueous solutions of acetic acid, sulfuric acid or chelating agents such as ethylenediaminetetraacetic acid or its mono-, di-, or tetra-substituted alkali metal salts.
The ethylenediaminetetraacetic acid compounds are effective but difficulties are encountered in using them to terminate polyphenylene oxide polymerizations. The free acid form of ethylenediaminetetraacetic acid is not soluble in aqueous or organic solvents to any great extent and must be added to the polyphenylene oxide polymerization reaction as a solid. This fact results in prolonged agitation cycles to assure intimate contact between the solid undissolved particles and the liquid reaction solution. The mono-, di-, tri- and tetra-sodium salts of ethylenediaminetetraacetic acid are of more practical importance for this extraction, since these salts are soluble in water to an appreciable extent and thus a shorter and less vigorous agitation cycle is required to extract the copper catalyst.
The rate of extraction of copper from the organic solution of a polyphenylene oxide by aqueous solutions of ethylenediaminetetraacetic acid salts appears to depend on two parameters: (1) pH of the aqueous solution of an ethylenediaminetetraacetic acid salt and (2) mixing efficiency of the two phases during extraction. With relatively mild agitation, as by hand-shaking or stirring in a beaker with a mechanical stirrer, the lower pH aqueous solutions of ethylenediaminetetraacetic acid exhibit faster extraction of copper than higher pH solutions of ethylenediaminetetraacetic acid salts, the rates following the order: di-sodium ethylenediaminetetraacetic acid (pH.about.7) &gt; tri-ethylenediaminetetraacetic acid (pH.about.9) &gt; tetra-sodium ethylenediaminetetraacetic acid (pH.about.11).
The extraction efficiency of a given ethylenediaminetetraacetic acid salt solution, defined as the weight fraction of copper extracted in a given time by a given mixing technique, therefore follows the same order. Increasing the efficiency of mixing the aqueous ethylenediaminetetraacetic acid phase with the organic solution of a polyphenylene oxide as by mixing in a Waring blendor or an homogenizer results in increased extraction efficiency of the higher pH solutions of ethylenediaminetetraacetic acid salts, as measured by the amount of residual copper in the polyphenylene oxide after precipitation of the polyphenylene oxide with a suitable anti-solvent, such as methanol.
It is desirable to utilize higher pH aqueous solutions of ethylenediaminetetraacetic acid salts for extracting copper from organic solutions; the two salts tri-sodium ethylenediaminetetraacetic acid and tetra-sodium ethylenediaminetetraacetic acid are less expensive than di-sodium ethylenediaminetetraacetic acid or ethylenediaminetetraacetic acid, and they do not extract the amine component of the catalyst as do the lower pH aqueous ethylenediaminetetraacetic acid salt solutions. However, these ethylenediaminetetraacetic acid salts (ethylenediaminetetraacetic acid (Na).sub.3 and ethylenediaminetetraacetic acid (Na).sub.4) require longer extraction times and/or improved mixing, as shown in the above discussion, to give low copper residues in polyphenylene oxide. Longer extraction times results in re-equilibration reactions of polyphenylene oxide to give reduced molecular weight, and are therefore undesirable. Improved mixing results in emulsification, making separation of the aqueous ethylenediaminetetraacetic acid-copper solution from the organic polyphenylene oxide solution difficult.
It has now been found that the rate of extraction of copper from polyphenylene oxide reaction solutions by aqueous high pH solutions of tri-sodium ethylenediaminetetraacetic acid and tetra-sodium ethylenediaminetetraacetic acid can be greatly accelerated by the addition to the two-phase extraction system of small amounts of quaternary ammonium salts, such as trioctylmethylammonium chloride, tetraethyl ammonium chloride and tetrabutyl ammonium chloride. Amounts of quaternary ammonium salt as low as 25 ppm based on the weight of organic polymer solutions are effective in this regard.
In the prior art quaternary ammonium salts have been employed in promoting the copper-amine oxidation of monovalent phenols in the preparation of polyphenylene oxide polymers. This is shown in U.S. Pat. No. 3,365,422 and in copending application Ser. No. 485,741. These processes employ the quaternary ammonium salts in the oxidation stage of the process and not in the polymer recovery stages.
The process of the present invention results in a reduced level of copper in the polyphenylene oxide product over that which would be obtained by the use of the quaternary ammonium salt alone or over the use of ethylenediaminetetraacetic acid or its salts alone, when the materials are employed in equivalent processing cycles.